<!DOCTYPE html>
<html lang="en">

<head>
    <meta charset="UTF-8">
    <meta name="viewport" content="width=device-width, initial-scale=1.0">
    <meta http-equiv="X-UA-Compatible" content="ie=edge">
    <title>Document</title>
    <style>
        * {
            margin: 0;
            padding: 0;
        }

        .search {
            position: relative;
            width: 178px;
            margin: 100px;
        }

        .sf {
            height: 30px;
            padding-left: 10px;
            outline: none;
        }

        .con {
            display: none;
            position: absolute;
            top: -43px;
            min-width: 178px;
            max-width: 440px;
            /* 让连续的数字自动换行 */
            word-wrap: break-word;
            border: 1px solid rgba(0, 0, 0, .2);
            box-shadow: 0 2px 4px rgba(0, 0, 0, .2);
            padding: 5px;
            font-size: 18px;
            line-height: 20px;
            color: #333;
        }

        .con::before {
            content: '';
            width: 10px;
            height: 10px;
            position: absolute;
            top: 25px;
            left: 18px;
            border-left: 1px solid rgba(0, 0, 0, .2);
            border-bottom: 1px solid rgba(0, 0, 0, .2);
            background-color: #fff;
            transform: rotate(-45deg);
        }
    </style>
</head>

<body>
    <div class="search">
        <div class="con"></div>
        <input type="text" placeholder="请输入您的快递单号" class="sf">
    </div>
    <script>
        const con = document.querySelector('.con')
        const sf = document.querySelector('.sf')
        sf.addEventListener('focus', function () {
            if (sf.value.trim() === '') {
                con.style.display = 'none'
            } else {
                con.style.display = 'block'
            }
        })
        sf.addEventListener('blur', function () {
            console.log(sf.value)
            con.style.display = 'none'
        })

        sf.addEventListener('input', function () {
            if (sf.value.trim() === '') {
                con.style.display = 'none'
            } else {
                con.style.display = 'block'
            }
            con.innerHTML = sf.value
        })
    </script>
</body>